In an automotive vehicle, a steering wheel is mechanically coupled to the wheels of the vehicle through a rack-and-pinion mechanism. An operator turns the steering wheel to rotate a steering column and thereby turn the vehicle wheels. Electric power steering assist systems, referred to as EPAS systems, have been developed that include an electric motor coupled to the steering mechanism to assist the operator in turning the wheels. A torque sensor detects torque in the steering column resulting from a turning command by the operator. The electric motor is actuated in response to the detected steering torque and reduces the steering force required by the operator to turn the vehicle wheels.
It is preferred to use a brushless motor in an EPAS system to optimize efficiency and reduce response time. The brushless motor comprises a stator that includes multiple electrical coils. Electric current conducted through the coils generates an electromagnetic field. A rotor having permanent magnets is positioned within the electromagnetic field. By regulating the voltage across the coils, the electric current is varied to rotate the electromagnetic field and thereby cause the rotor to rotate. The current within the electric motor is characterized by space vectors that feature a direct axis current component, referred to as Id, and a quadrature axis current component, referred to as Iq. In general, the control module for the EPAS system determines an Iq to provide a torque assist to the operator and calculates the voltages to the coils to achieve the desired Iq. Motor torque efficiency is optimized when the Id is zero, so the control module calculations are based upon 0 Id.
The speed of the motor is determined by the mechanical coupling between the steering wheel and the steering mechanism. That is, the electric motor does not turn the vehicle wheels faster or slower than the steering command from the operator. The control module regulates the rotation of the electromagnetic field, and thus the rotor, to match the steering command. The design parameters of the motor determine a maximum design motor speed. Under certain conditions, the operator may seek to rotate the steering wheel faster than the maximum design speed. Such conditions may arise, for example, when making evasive maneuvers. Under such conditions, the motor does not provide the desired assist and may even act as a drag hindering the rapid turning of the wheels, resulting in a noticeable increase in steering force required by the operator.
Therefore, there is a need for an improved EPAS system wherein a brushless electric motor is controlled by space vector modulation and wherein the system is capable of responding to a rapid steering command by the operator by temporarily increasing the motor speed above the maximum design speed for the motor.